a,b,c=10.0,20.0,3.0
d=capi.Fcal(a,b,c)　#Calculated in c language
print(a*b/c) #Calculated with python
print(d)

Execution result
capi>python capi.py
66.66666666666667
66.66666666666667

#include <Python.h>
static PyObject* Fcal(PyObject* self, PyObject* args){
    double a,b,c,d;
    if (!PyArg_ParseTuple(args, "ddd",&a,&b,&c)){
        return NULL;
    }
    d=a*b/c;
    return Py_BuildValue("d", d);
}
// Function Definition struct
static PyMethodDef myMethods[] = {
    { "Fcal",Fcal, METH_VARARGS, "Fcal a*b/c"},
    { NULL }
};
static struct PyModuleDef myModule = {
    PyModuleDef_HEAD_INIT,"myModule","C API Module",-1,myMethods
};
PyMODINIT_FUNC PyInit_myModule(void){
    return PyModule_Create(&myModule);
}

double a,b,c,d;
    if (!PyArg_ParseTuple(args, "ddd",&a,&b,&c)){
        return NULL;
    }
    d=a*b/c;
    return Py_BuildValue("d", d);

from distutils.core import setup, Extension
setup(name = 'myModule', version = '1.0.0',  \
   ext_modules = [Extension('myModule', ['capilib.c'])])

capi>python setup.py install

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